For those who choose to have children early in life, the trade-off may involve more than just money

Babies cost you dearly. Put aside the romantic images of first steps and bike rides and tearful college graduations, and parenthood is a series of transactions, investments and calculations of risk vs. reward. And these are not just about money. Your children will cost you thousands of dollars, sure, but also chunks of your youth, middle and old age, physical stamina and, at least for many women, career opportunities.

Of course, all this is true at any age. But to extend the financial metaphor, deciding to have her family while she's in her 20s changes a woman's investment horizon. A younger...